1. Field of the Invention
The present invention relates generally to thermal printing devices. More specifically, the present invention relates to a thermal printing device, a method for printing a multicolored image using the printing device and a system for printing multicolored images.
2. Description of Related Art
Various conventional printing devices include a printing head that is capable of transferring a colorant to a substrate. Several different techniques may be used for the transfer of colorant, including ink jet, electrostatic toner transfer, and thermal transfer. Printing devices using these techniques can print a single or more than one color, and may print onto individual or continuous sheets that may be opaque or transparent.
Users of printing devices now demand printing of photographic quality so that they can, for example, print digital images captured from digital cameras. The desire for photographic quality, full-color images has forced conventional, colorant-transfer printing technologies to evolve to their limits. Such technologies have, in some cases, proved to be less than satisfactory for photographic printing.
Direct thermal printing provides an entirely different method for forming images on an imaging material, which may be in the form of an individual sheet of a specific size, e.g., 4×6 inches or a continuous sheet. Typically, the imaging material includes a substrate, or carrier, and a plurality of color-forming layers can be arranged on one side of the substrate or one or more color-forming layers can be arranged on each side of the substrate. A direct thermal printing device includes no ink, toner, or transfer ribbon, but simply a printing head for heating the imaging sheet itself. The imaging material for use in direct thermal printing contains at least one dye or dye precursor that changes color when heated. Examples of direct thermal printing systems are disclosed in, for example, U.S. Pat. No. 6,801,233 B2 assigned to the assignee of the instant application.
Imaging materials for direct thermal printing devices that are intended to produce multicolored images may be transparent, and may include at least one color-forming layer on each surface. Each color-forming layer on one side of the substrate forms an image in at least one color, while each color-forming layer on the other side of the substrate forms an image in at least another color. Images are formed by heating each side of the imaging material with a thermal head or other heating device, which can apply heat in an imagewise pattern. The images formed on each side of the transparent substrate are viewed together from one side of the imaging material to present to the viewer a composite, multicolored image. For this reason, the images on either side of the substrate must be substantially the same size and substantially in perfect registration with each other. In conventional printing onto an opaque imaging sheet, on the other hand, there is no need for the images on two sides of the sheet to be the same size or in registration.
Several methods for printing on both surfaces of a direct thermal imaging material have been proposed. For example, U.S. Pat. No. 4,962,386 discloses a printing device with an extremely complex mechanism for rotating the substrate such that both surfaces can be exposed to a print head sequentially. In U.S. Pat. No. 6,601,952 a method is disclosed for rotating an entire recording unit to print on the second surface of an imaging material. Another method for imaging both surfaces of a direct thermal imaging material employs two print heads, one of which heats one side of the imaging material, while the other heats the opposite side. Each of these prior art methods for printing involves complex arrangements that may be high in cost or difficult to maintain.
Accordingly, there is a need for a direct thermal printer with a simplified construction that can overcome the deficiencies of the prior art printers, including achieving more accurate registration of images printed on both surfaces of a direct thermal imaging material.